The present invention relates to the fabrication of integrated circuits. More particularly, the invention provides methods and apparatus for preventing undesired aluminum deposition build-up that occurs during substrate processing. The present invention is particularly useful for aluminum chemical vapor deposition processing, but may also be useful for other types of substrate processing.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a metal film, such as an aluminum film, on a semiconductor substrate. Processes to form an aluminum film are often used in deposition of a metal layer or as part of the deposition process of metal interconnects between one metal layer and either the substrate or another metal layer. Conventionally, an aluminum film is formed on a substrate either by a sputter deposition process or by a chemical vapor deposition (CVD) process. In an exemplary sputter deposition system, a target (a plate of the aluminum material that is to be deposited) is connected to a negative voltage supply (direct current (DC) or radio frequency (RF)) while a substrate holder facing the target is either grounded, floating, biased, heated, cooled, or some combination thereof. A gas such as argon is introduced into the system, typically maintained at a pressure between a few millitorr (mtorr) and about 100 mtorr, to provide a medium in which a glow discharge can be initiated and maintained. When the glow discharge is started, positive ions strike the target, and target aluminum atoms are removed by momentum transfer. These target atoms subsequently condense into a thin aluminum film on the substrate, which is on the substrate holder. In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to form the thin aluminum film over the surface of the substrate being processed. One exemplary thermal aluminum CVD process deposits aluminum over a semiconductor substrate from a process gas that includes dimethylaluminum hydride (DMAH) and a carrier gas such as argon. As is well known, aluminum CVD is a "selective" process in that aluminum prefers to deposit on a metallic surface. Once the deposition of aluminum on a metallic surface has begun, the deposited aluminum, itself a metal, supports further growth of aluminum. Although aluminum deposits on a metallic surface more rapidly than on an insulating surface, aluminum also will deposit on an insulator after a long incubation time.
One problem that arises during aluminum sputtering or aluminum CVD processes is that unwanted aluminum deposition occurs in the processing chamber and leads to potentially high maintenance costs. During aluminum deposition of a single wafer, some aluminum deposition is likely to occur on the heater and/or process kit if these are made of metal and even if these are partially or wholly made of an insulating material. During subsequent wafer depositions, this excess aluminum growth on the heater and/or process kit will accelerate until a continuous aluminum film is grown on the heater and/or process kit. Over time, this unwanted aluminum deposition, unless removed, may interfere with the performance of the processing chamber. To prevent chamber performance from being adversely affected by the accumulated aluminum, portions of the processing chamber (such as the heater, shadow ring, and/or gas distribution faceplate) may need to be removed and replaced periodically. Depending on which and how many parts need replacing and the frequency of the replacement, the cost of maintaining the deposition system can be very high.
This unwanted aluminum deposition problem is fairly easily solved for sputtering deposition systems. With sputtering deposition, aluminum deposition is limited to areas of the chamber which have an uninterrupted "line of sight" to some part of the target. Therefore, sputtering systems often utilize a shield ring or clamp ring to accumulate excess aluminum deposition and to protect the heated substrate holder from unwanted aluminum deposition. Typically about 3000-6000 wafers may be processed (about 0.3-0.6 mm total deposition) with an aluminum sputtering deposition system before excess deposition starts to interfere with the system's performance. Then, only the shield ring or clamp ring and one or two other process kit parts, not including the expensive heater, need to be removed and replaced to remove unwanted accumulations of aluminum in the sputtering system.
However, the problem of unwanted aluminum deposition is not so easily solved for aluminum CVD systems. Unlike sputtering deposition, aluminum deposition in CVD systems is not limited to certain areas of the chamber. With CVD, aluminum deposition can occur on any hot surface including the shadow ring and the heater, because the reactive gases can diffuse everywhere in the processing chamber, even between cracks and around corners in the processing chamber. Typically, about 5000 wafers may be processed (about 0.5 mm total deposition) with an aluminum CVD system before excess deposition starts to interfere with the system's performance. Then, the heater and other process kit parts (such as the shadow ring and gas distribution faceplate) may need to be removed and replaced to remove unwanted accumulations of aluminum in the CVD system. If not protected in some way from unwanted aluminum deposition, the heater and other process kit parts would become expensive consumable items that need to be replaced periodically in order to prevent chamber performance from being adversely affected.
Typical solutions to similar unwanted deposition problems faced in other non-aluminum CVD processes have been considered but found inadequate for solving the problem in aluminum CVD process. With these non-aluminum CVD processes, similar problems with excess deposition on the inside surfaces of the chamber have been minimized by utilizing reactive plasma cleaning and by using process kit parts constructed of materials resistant to the etching gas used in the cleaning. In these CVD processes, a reactive plasma cleaning is regularly performed to remove the unwanted deposition material from the chamber walls, heater, and other process kit parts of the processing chamber. Commonly performed between deposition steps for every wafer or every n wafers, this cleaning procedure is performed as a standard chamber cleaning operation where the etching gas is used to remove (etch) the unwanted deposited material. Common etching techniques include plasma CVD techniques that promote excitation and/or disassociation of the reactant gases by the application of RF energy to a reaction zone proximate the substrate surface. In these techniques, a plasma of highly reactive species is created that reacts with and etches away the unwanted deposition material from the chamber walls and other areas. However, with aluminum CVD processes, etching gases useful for etching aluminum contain chlorine, which is very reactive to many, if not most, of the materials which make up the chamber, heater, and process kit parts of the processing chamber. Therefore, using chlorine-containing etching gases makes it virtually impossible to effectively clean excess CVD aluminum film from an aluminum heater without also damaging the heater in the cleaning process. Even if the heater and/or process kit parts are constructed of materials resistant to the chlorine-containing etching gas used in the cleaning, unwanted aluminum deposition could still occur although at a slower rate. Thus, maintaining chamber performance may result in damage to expensive consumable items which need to be replaced frequently as a result.
From the above, it is seen that a method and apparatus are needed to minimize unwanted aluminum deposition on the heater, process kit parts, and chamber walls in aluminum CVD systems without causing damage to these components. Further, it is desirable that the method and apparatus extend the life span of these components to thereby reduce the need for and/or frequency of replacing expensive consumable items.